Aerial applications of ultra-low-volume insecticides to control the vector of Japanese encephalitis in Korea

Bull. OrgW mH Org.Sante 1973, 49, 353-357 Aerial applications of ultra-low-volume insecticides to control the vector of Japanese encephalitis in Korea L. S. SELF 1, H. I. REE 2, C. S. LOFGREN 3, J. C. SHIM 2, C. Y. CHOW 4, H. K. SHIN 2, & K. H. KIM 5. As a suitable emergency measure to arrest epidemics ofJapanese encephalitis in Korea, the ultra-low-volume method of spraying insecticide to control the mosquito vector Culex tritaeniorhynchus has been tested in 2 successive years over a 16-km 2 area, utilizing a large fixed-wing aircraft. Malathion concentrate applied at 0.36 litres/ha gave insufficient control of the parous (infective) females, and no reduction in total numbers of this species. Fenitrothion concentrate applied at 0.45 litres/ha resulted in a 77-87 % reduction in total numbers and an 87-98 % reduction in parous females over a 4-day period. Japanese encephalitis (JE) is of great public health importance in Korea, where the only known vector is Culex tritaeniorhynchus summorosus. Numerous cases are reported every year and epidemics have been frequent in some years, the highest incidence being in children, e.g., 3595 cases and 957 deaths in 1966 (2). Cases occur from late July to September, with the peak in August. Recently, the greatest number of cases have occurred on the outskirts of the growing cities of Korea. Whereas the hazard of JE has been reduced by vaccination programmes and by the increasing use of insecticides in the rice-fields, there is a need to develop an emergency method for coping with. well defined local outbreaks. The aerial spraying of certain organophosphorus insecticides in virtually undiluted form, termed the ultra-low-volume (ULV) method, has proved successful in stopping an epidemic of St Louis encephalitis at Dallas, Texas (1) and in 1 WHO Japanese Encephalitis Vector Research Unit, Seoul, Korea. Present address: WHO Vector and Rodent Control Research Unit, P.O. Box 302, Djakarta, Indonesia. ;' Entomologist, National Institute of Health, Seoul, Korea, seconded to WHO Japanese Encephalitis Vector Research Unit. ' WHO Short-term Consultant, from Entomology Re- search Division of Department of Agriculture, Gainesville, Fla., USA. ' Regional Adviser on Vector Biology and Control, WHO Western Pacific Regional Office, Manila, Philippines. ' Director, Department of Virology, National Institute of Health, Seoul, Korea. controlling the vector of dengue haemorrhagic fever at Bangkok (4). Two large-scale trials were therefore performed on the outskirts of Pusan, Korea, one with 95% malathion in 1971 and the other with 95% fenitro- thion in 1972, using a C-46 aircraft operated by the Korean Air Force. METHODS AND MATERIALS The test area Sasang, a 16-km2 area immediately southwest of Pusan delimited by the Nak Tong river on the west and north and by mountains on the east and south, was selected. It consists of rice-fields, with small villages becoming " ribbon-development " along its eastern side. The farmers maintain pigs and cows and grow vegetables, while at the same time city-type buildings are beginning to appear in the area. There are occasional marshes with reeds 2 m high, and the rice at the time of the trials (August) was 0.8 m high. The prevailing winds in the evening were from south to north at that time. Aircraft installation The C-46 transport aircraft was fitted with a boom-and-nozzle assembly, kindly provided by the US Department of Agriculture Laboratory, Gaines- ville, Fla. and similar to that described by Lofgren et al. (5), except that the boom was longer; it carried 3126 - 353 - 354 L. S. SELF ET AL. 10 Teejet nozzles with 8008 flat-fan tips. With a pump pressure of 2.412 x 106 Pa a it emitted ULV insecticide at the rate of 26.5 litres/min. The droplets produced were found, by assay with slides coated with magnesium oxide, to have a volume median diameter of 56 ,um. Application procedure A line of stakes, 183 m apart in the malathion trial and 152 m apart in the fenitrothion trial, was established across the northern end of the test area, to provide stopping points for a marker vehicle to indicate the starting points of each spray run. In the malathion trial, in which there was ground-air radio communication, there were similar points at the south end of the spray run; in the fenitrothion trial, with no radio, the pilot flew on a 1890 bearing. Applications were made at a speed of 240 km/h from a height of 46 m. Application performance The malathion treatment was applied on 7 August 1971 during the period between 1800 and 1915 h; the wind was from the south to southwest (180-200°) at a speed of 14 km/h, decreasing to 6 km/h. The fenitrothion treatment was applied on 5 August 1972 between 17 h 50 and 19 h 30; the wind was from 190-200' at a speed of 16 km/h, decreasing to 6 km/h. Since it was evident that in the malathion trial the ULV spray had been extensively displaced northwards at the target area, in the fenitrothion trial the spray was not turned on over the starting-point, but was delayed by 45 s (equivalent to 3.0 km), decreasing to 35 s (2.2 km). The spray was turned off 1 min 40 s later, by which time the aircraft was turning right to avoid the southern mountains; in this way the spray could drift back to cover the southern part of the test area, which was missed in the malathion trial. In the malathion trial 568 litres were expended over 15.8 kM2, a dosage equivalent to 0.36 litres/ha; in the fenitrothion trial, 723 litres were expended over 16.2 kM2, a dosage equivalent to 0.45 litres/ha. Biological assessment Fixed collecting stations were established, 3 in the malathion trial and 5 in the fenitrothion trial. Ani- mal-bait collections of adult Culex tritaeniorhynchus were taken off cows (in one case in the malathion trial, a pig) by one man working during the hour a = 2.46 kgf/cm2. between 20 h 00 and 21 h 00. Animal-shelter collec- tions were made by 1 man during the quarter-hour between 18 h 00 and 18 h 15. Light-trap collections were made either with CDC a1 battery-operated traps or with Nozawab black-light line-operated traps. Parous rates were determined from collections of 126-238 mosquitos taken from animal bait on each of the post-spray nights. RESULTS The results of the malathion trial (Table 1) show that the light-traps and the animal-bait collections revealed little or no reduction in the numbers of Culex tritaeniorhynchus. However, if the proportion of the parous mosquitos, as an index of the possibly infective mosquitos, is taken into account, it is seen that some reduction has been achieved in the num- bers of possible vectors, at least for the first 48 h after the ULV application. The results of the fenitrothion trial (Table 2) show that animal-bait collections revealed a reduction in numbers amounting to an average of 80% for the first 96 h after the ULV application. The collections from animal shelters were reduced by 70% during this period, while the light-trap collections were reduced by only about 30%. If the proportion of parous mosquitos is taken into account, the collec- tions from animals and their shelters show a reduc- tion of more than 90% in the numbers of possibly infective mosquitos during the first 72 h after spray- ing. Application of the parous rates, obtained from animal collections, to the light-trap results also indicates a significant reduction, which may in fact have been greater than that shown, since it is likely that light-traps attract mainly the newly emerged nonparous mosquitos. Sampling cards dyed with Oil Red, placed horizontally on a line across the middle of the test area and in its southeast corner, indicated that the coverage with fenitrothion was uninter- rupted, ranging from 1 to 7 droplets/cm2, and that some spray reached the far southeast corner. In the malathion trial, field-collected C. tritaeniorhyn- chus held in 18-mesh nylon cages at 14 stations on a line across the test area showed 100% mortality in exposed positions and an average of 68% mortality in protected situations such as pigsties and under vegetation. Larvae of this species contained in alumi- a Communicable Disease Center, now Center for Disease Control, Atlanta, Ga., USA. b Nozawa Mfg. Co., 18 Kita-machi, Isesaki City, Gumma Prefecture, Japan. Table 1. Control of infective Culex tritaeniorhynchus by ULV malathion at 0.36 litres/ha Numbers of females at 3 sites Proportion No. of tReduc-Days of parous of parous tion ipost-spray II Average females females parousfemales females Animal-bait collections/man-hour prespray 487 108 265 287 0.30 86 1 439 337 103 293 0.08 24 72 2 219 370 193 261 0.19 50 42 4 694 290 138 374 0.20 75 12 Light-trap coll ections/trap-night prespray 871 185 219 408 0.30 124 1 870 371 306 516 0.08 41 67 2 416 3 0 140 0.19 27 78 4 891 0 518 470 0.20 94 24 Table 2. Control of infective Culex tritaeniorhynchus by ULV fenitrothion at 0.45 litres per ha Days Numbers of females at 5 sites Proportion No. of % Reductionpost-spray of parous parous in parouspost-spray I I IlIl IV V Average females females females Animal-bait collections/man-hour prespray 552 425 526 163 614 456 0.24 114 1 14 51 44 31 170 62 0.03 2 98 2 160 61 30 36 244 106 0.08 8 93 3 95 88 78 63 144 94 0.08 8 93 4 121 28 107 20 110 77 0.19 15 87 5 93 703 230 4 148 238 0.11 26 77 6 260 668 290 20 558 359 0.17 61 46 Animal-shelter collections/man- hour prespray 72 92 4 20 176 72 0.24 17 1 20 16 4 0 104 28 0.03 1 94 2 16 0 4 0 20 8 0.08 1 94 3 56 20 4 0 64 28 0.08 2 88 4 12 0 0 4 80 20 0.19 4 76 5 76 304 12 0 216 120 0.11 13 24 6 12 120 8 0 284 84 0.17 14 18 Light-trap collections/trap-night prespray 83 639 246 254 63 257 0.24 62 1 19 384 20 422 200 209 0.03 6 90 2 64 277 44 77 176 128 0.08 10 84 3 142 406 20 177 356 250 0.08 20 68 4 174 321 158 1 6 247 183 0.19 35 44 5 120 969 59 78 277 301 0.11 33 47 6 439 1956 21 627 948 798 0.17 136 0 3 L. S. SELF ET AL. nium bowls placed under vegetation at each of these stations showed 100% mortality after the malathion spray. In the malathion trial, field-col- lected C. tritaeniorhynchus held in 18-mesh nylon cages at 14 stations on a line across the test area showed 100% mortality in exposed positions and an average of 68% mortality in protected situations such as pigsties and under vegetation. In the fenitro- thion trial, similar assessments showed average mor- talities of 99% in exposed situations, 86% inside animal shelters, 83% under light vegetation, and 72% under thick vegetation. Assessment of the larvae found in rice-fields in the malathion trial revealed 100% control in the first 3 days, declining to 88% on the 5th day after spraying. Assessments in the 3 days following the fenitrothion trial revealed 100% control in ditches but only 67% reduction in the marshes with their very high vegetation. In the malathion trial, larvae of C. tritaeniorhynchus in aluminium bowls placed under vegetation in a line across the test area showed 100% mortality. In the fenitrothion spray, larvae placed in thin-mesh nylon cages in the natural breeding sites suffered 88% mortality in rice-fields, 80% in open ditches, and 67% in marshes. DISCUSSION The superior results obtained with fenitrothion are partly connected with the fact that C. tritaeniorhyn- chus is more susceptible to fenitrothion than to malathion, as shown by the WHO standard test for adults exposed to papers impregnated with these insecticides (Mitchell & Chen, unpublished observa- tions, 1972; Self et al., unpublished observations, 1973). They are also partly connected with the non- coverage of the southern part of the test area in the malathion trial. From the figures for each collecting station (Table 2), it can be seen that the best control for the longest period was obtained at the central sites (II, III, and IV). Light-trap III was a CDC visible-light trap, while the others were black-light traps. The black-light traps proved particularly attractive; the captures on the night of the fenitrothion application (not shown in Table 2) were as follows: I 122, II 1064, III 17, IV 448, and V 567. As these captures are double those on the night before the spray, it appears that black light was particularly attractive to the adult C. tritaeniorhynchus activated by the spray, and remained more attractive than visible light to the populations surviving on subsequent nights. It will also be noted from the figures for the malathion trial (Table 1) that light-traps I and III, which were black- light traps, obtained higher catches than the visible- light trap II. The fenitrothion application (Table 2) resulted in a nearly 80% reduction in adult numbers for the first 4 days of the spray, and an 87-98% reduction in parous females, as judged by collections from animal bait. The reduction in adults resting in animal- shelters was slightly better than 70% for this 4-day period, and the reduction in those coming to light- traps was only 25 %. Although the parous rate of the mosquitos taken in the light traps was not assessed, it would be reasonable to assume that it was very low indeed, most of the captures being new emergents from the pupae that naturally survived the spray application, and from an unknown proportion of immigrants from the surrounding unsprayed area. Probably the animal shelters also contained an im- portant element of newly-emerged adults. Thus although the results with these ULV insecticides at 0.45 litres/ha appeared superficially disappointing compared with those previously obtained against pest and vector species in the USA (3), it was in the numbers of those adults that were due for a subse- quent, possibly infective, blood meal that a signifi- cant reduction (87-98%) was obtained for a 4-day period by the fenitrothion application. It should be noted that this net result was obtained by military aircraft and personnel in the absence of radio communication. This result was also obtained in spite of two important and unavoidable handi- caps. First, the lines of flight must of necessity parallel the mountain and valley systems, with which the prevailing winds are also parallel. Second, the aerial application must be completed before sunset (night operation being too dangerous in such moun- tainous terrain), while the mosquitos do not usually leave their cover and become active until 1-1.5 h after sunset. It is thus evident that although the use of ULV sprays from a helicopter would achieve better penetration and coverage, the use of drift- spraying from a large military fixed-wing aircraft is an effective procedure that can be carried out very quickly at any place in Korea where a JE epidemic might be recognized to be in progress. 356 ULV INSECTICIDES AGAINST JE VECTOR 357 ACKNOWLEDGEMENTS Special thanks for their contribution to these large-scale trials are due to the following: Dr S. W. Lee, Chief, Public Health Section, Ministry of Health and Social Affairs; Cols S. U. Kong and Y. K. Koh, Lt-Col K. H. Cho, Majors P. S. Kim, S. H. Sang, and Y. J. Chung, and Captains S. Y. Hong and H. J. Kim of the Republic of Korea Air Force; Col T. Oliver, Camp Hialeah, Pusan, and Captain M. Bosworth, 5th Preventive Medicine Unit, Seoul; the Pusan Veterinary Diagnostic Laboratory and the Pusan City Hygienic Laboratory; Mr J. Hattori, Sumitomo Chemical Company, Osaka; Mr T. Keim, Lee County Mosquito Control District, Fort Myers, Fla., USA, acting as WHO Consultant; Drs P. H. A. Jolivet and T. Okuno, Messrs H. K. Hong, D. W. Park, and K. W. Lee, and Miss S. H. Yun of the staff of the WHO Vector Ecology and Control Research Unit, Seoul. R1ESUMt APPLICATIONS AtRIENNES D'INSECTICIDES SOUS ULTRA-FAIBLE VOLUME POUR LUTTER CONTRE LE VECTEUR DE L'ENCAPHALITE JAPONAISE EN CORtE On a evalu6 l'efficacit6 de pulverisations a6riennes d'insecticides organophosphor6s sous ultra-faible volume dans la lutte contre Culex tritaeniorhynchus, vecteur de l'enc6phalite japonaise en Cor6e. Dans la r6gion de 1'essai, d'une superficie de 16 km2, pres de Pusan, le malathion a ete utilis6 le 7 aouit 1971 a la dose de 0,36 litre/ha et le f6nitrothion a la dose de 0,45 litre/ha le 5 aout 1972. La pulv6risation a ete effec- tuee par un avion C(46 volant a 240 km/heure i 46 m d'altitude entre 18 h et 19 h 30. Le diametre moyen des gouttelettes d'insecticide 6tait de 56 ztm. L'effet a 6t6 mesur6 par la collecte de moustiques dans les abris pour animaux, sur pieges animaux et par pieges lumineux. On a aussi expos6 des moustiques encages. L'application de malathion n'a pas reduit le nombre total des C. tritaeniorhynchus, bien que les populations de femelles pares, donc potentiellement infectantes, aient diminu6 dans la proportion de 40-80% durant les 2 pre- miers jours suivant la pulverisation. L'application de f6nitrothion a entrain6 une r6duction de 77-87% de la population de moustiques et de 87-98% de celle des femelles pares pendant les 4 jours suivant le traitement. D'apres les r6sultats obtenus sur les moustiques encag6s, les insecticides ont et6 efficaces A 100% sur les moustiques tres exposes, alors que sur les sp6cimens b6neficiant d'un abri, 1'effet destructeur du malathion n'a et6 que de 68% et celui du fenitrothion de 72-86%. Ces r6sultats montrent que l'application de fenitrothion A la dose de 0,45 litre/ha, effectu&e i partir d'un grand avion de transport, assure la destruction quasi complete des adultes infectants d'une population de vecteurs pen- dant les 4 jours suivant le traitement. REFERENCES 1. KILPATRICK, J. W. & ADAMS, C. T. Proc. Ann. Conf. Calif. Mosquito Control Ass., 35: 33 (1967). 2. KONO, R. & KIM, K. H. Bull. Wld Hlth Org., 40: 263- 277 (1969). 3. LOFGREN, C. S. Ann. Rev. Ent., 15: 321-342 (1970) 4. LOFGREN, C. S. ET AL. Bull. Wid Hlth Org., 42: 15-25 (1970). 5. LOFGREN, C. S. ET AL. Bull. Wld Hlth Org., 42: 157- 163 (1970).